oh man, now I really want to try that! I had so many of these HD5As that I was giving them away at one stage - now I wish I'd kept more than just one. I wonder if a standard HD1D could be used if one replaced the diaphragm cover with the metal type, and replaced some of the other parts with race items...would there be any benefit in retaining the choke?

I may look at removing those round spigots for a better look inside the extended tracts.

Yep, people don't seem to like the Tillotson much... still, I used to run my KHK with a pair of them some time ago before I went for the twin Linkert DC... I got rid of the flanges to tighten the assembly width, it did start OK, ran very well...

One thing with Tillotsons, don't try to run any diaphragms that look slightly deformed or hardened by age. NOS ones are still around, as well as rebuilt kits. racers did not use the accelerator pump/plastic cover, but the venturi have to be moded to race specs if you delete the pump. This also makes the choke unusable. I have the drgs if you want, they are in the KR/KRTT/XLRTT manuals, repros on eBay...

Other problems with twin Tillotsons is the tuning of the gas inlet valve. Springs vary in tension, you'll need more than 2 to find a set that'll give the same performance!!!

Also fit a small cable tie attached near the underneath to one of the screws (or always carry a match!!!), invaluable tool to flood the carb for starting by poking it in the chamber through the bottom vent hole to lift the diaphragm...

Quite a dog-leg between the new port alignment and the original (and proven effective) port location.Which then begs the question:Is the benefit (if any) of dual carbs of greater value than the loss of efficiency from re-locating the port?

Yes the set up is fairly crude, although the inside is a smooth arc all the way around that is formed with braze and overlaid with some kind of putty. Although the workmanship on the outside looks rough, the inside means business and there was some nice work done in there with what look like modified guides etc.

I like to think of it in the context of the race season back in '70, with the guys working flat out in the shop to wring as much power out of the iron heads as the racers moved around the country to the various events - stressful times I imagine. Per Girdler's book the design went through several evolutions after this one, eventually moving to the XR design approaching what we see today. You'd definitely know more about it than I do (I tend toward the earlier stuff) and am still learning about all of that), but I understand that the RHS dual carb layout moved the ports around the head to the current config in following seasons.

Whatever the story, someone put a lot of time and effort into porting these heads a long time ago - I don't see them as being an optimum design, but rather a slice of history in the engine's development. Neat stuff!

BTW I think the dark horses or often overlooked bikes of this era - the 52 and very early 53 K model, the iron XR, etc are great bikes that really capture a moment in history from the era. When you look at a very early original K model they're really spindly and the finish is quite cheap (think korean War era restrictions). Cool in a simple early 50s way.

The iron XR must be a brute to ride, but it's the same simple raw design from a snapshot of a year or two where things must have been moving very fast for everyone. Every one of the few machines was probably different, set up for particular riders or modified locally by delears or races. You know how it goes, the bikes are run in a config that is evolved or modified over a few seasons then the bikes are passed down through the classes or on to newer riders when it's time for an upgrade. Or they're shelved and become spare machines or project bikes. Some could become hillclimbers, or dirt trackers, or drag bikes, or street trackers, or piles of parts and basket cases as people move on or change interests.

I like the sound of Mike's project to build and experiment with the dual carb KR & dome pistons - that will be an interesting engine and fettling project to hear about!

I agree. One of the good things about the vintage and classic bike scene in the UK is that it's fairly small geographically, and you tend to meet most of the people and see most of the bikes over time. There are some highly unusual vintage racers around, factory developed and privately developed, and it makes for a lot of extra interest in the sport

Shoot, a man could have a good weekend in Dallas with all that stuff...

Re-thinking my post - if enough development time was invested, it may be possible to incorporate a single radius between the new port entry and the seat to induce swirl, and actually out-perform the original port. No idea how to analyze it without a bench, though.